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Old 09-16-2010, 11:32 PM   #1 (permalink)
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Engine Temp and wear

Hi all. Been a while since I posted, but I'm in my junior year in mechanical engineering and just finished discussing junior/senior design projects with one of my professors. We refined the idea down to a project on using waste heat to power some sort of steam engine (quasiturbine, cyclone, something cheaper?) but one of the hurdles we want to overcome is the operating temperature of the radiator.

That being said, yes, the exhaust is fairly fertile ground for harvesting heat, but the coolant is easier to control the flow for a heat exchanger, though the final product may well cover both.

Anyway, we'll be using my Honda Civic, which operates around 195 degrees F, which is simply not hot enough to boil pure water. We discussed raising the operating temperature to a higher level, but can't find any specific data on just exactly how this will affect engine life. Thermal expansion and reduction of oil viscosity would be the key things to look at, however we're aiming for something like 220-240 degrees. Incidentally, if we raise the engine temp, overall efficiency should increase because of the difference in exergy and all

From what I know of the aluminum and steel, their coefficients of thermal expansion are relatively small, especially for a change of only 30-50 degrees F. So, I don't think that will really be an issue of concern.

Has anyone seen data regarding the loss of oil life at temp? I've seen some viscosity figures like SAE 30 at 190F is about the same viscosity as SAE 40 at 240 degrees or some such, but nothing about breakdown or even multi-viscosity oils. We are in FL, so multi-viscosity is not critical as the ambient temp is rarely below 50 degrees, but I'd still feel better having that option.


Of course, if all else fails, we may just use the radiator as a warmer and send the near-boiling water over a specially constructed exhaust manifold/heat exchanger.

But I would love to get some input from you guys about the overall idea, as well as any pertinent information you may have regarding oil life vs engine temp.

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Old 09-16-2010, 11:41 PM   #2 (permalink)
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Here's a 22-page thread on the topic: http://ecomodder.com/forum/showthrea...wer-13727.html , and I've posted a few thoughts in that thread that are relevant to this one.

I think you're crazy for trying to run a heat engine between Th=100°C and Tc=30°, especially considering your working fluid will achieve an even smaller range of temperatures.

Also, if it must run off of engine coolant, what about a working fluid other than water? Maybe alcohol?
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Old 09-17-2010, 12:10 AM   #3 (permalink)
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Raising the temperature of the coolant above 100 degrees C is a bit worrying. I think my car tries to run its coolant at 85C

I think antifreeze changes the coolant temperature boiling point up to 135 C but you will have to make sure you put enough antifreeze in the car, the wrong ratio will give you lower temperature of boiling.
I do like robertsmalls idea of changing the working fluid to alcohol or some lower boiling point fluid
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Old 09-17-2010, 12:39 AM   #4 (permalink)
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A diesel engine can manage a much higher operating temperature, but it is not because of the oil viscosity. The problem that gasoline engines prior to HCCI have is pre-ignition/detonation. To raise the temp of a gasoline engine's coolant to 230 to get a sufficient heat transfer and, therefore, significant amount of water vapor from the boiling, you would encounter significant combustion chamber temperatures. This can be overcome by a number of methods: direct injection, use of a lower boiling point fluid for your energy transfer (steam engine), and giving up on the idea of a steam engine.

As for the oil temp, you could add an additional cooler, but if my memory serves me at all, oil temp is higher than coolant temp, a potential area to harness energy from.

I would also avoid exhaust heat exchangers, as I foresee some issues in power loss do to the exhaust cooling too soon and the gases not acting as intended, and also you would have to locate it post-catalyst. Good luck with the challenges.
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Old 09-17-2010, 01:02 AM   #5 (permalink)
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Quote:
Originally Posted by autoteach View Post
As for the oil temp, you could add an additional cooler, but if my memory serves me at all, oil temp is higher than coolant temp, a potential area to harness energy from.
...pressure cap + antifreeze = 245°F

...without separate radiator/cooler, typical engine oil = 230-240°F, hot enough to keep water "out" of the engine oil...remember, it's washing down 400°F cylinder walls on its way back to the sump.
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Old 09-17-2010, 02:17 PM   #6 (permalink)
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If you use a closed system on the heat exchanger side that can hold a vacuum it will reduce the boiling temp, but as pressure rises from the boiling water it will drop the vacuum increasing the boiling temp. So you would need to place a large enough vacuum on it to over come that.
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Old 09-17-2010, 05:07 PM   #7 (permalink)
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Vacuum is a good idea. What kinds of pumps could be found easily to pull a vacuum on steam, though? I mean, the engine produces tons of vacuum, but then it would pull all the steam through the intake which is probably not good. I'm wondering if a washer fluid pump would be able to put up with that kind of temp or even be able to pull enough.

On a side note, I was able to get an old 2-stroke to turn over on air earlier, so I'm gonna high-tail it over to home depot/radio shack to grab a reed switch and solenoid valve to see if I can get it running on compressed air, then on steam. Seems pretty doable, though those solenoids have a short life. Once I have it running on steam, I'll get some pics/video and try to get it turning an alternator or whatever I have around to bog it down. Baby steps, though. Today I'm going to try to get it running on compressed air.

So, I'm looking through my thermodynamics text book at the saturation values of water, r-134a, and r-410 and at 190F, 134a has a saturation pressure of about 450 psi. My table for 410 stops at 160F, but it's pressure is 710 psi. Water at boiling is only 14 psi. I'm probably wrong in inferring that this means a similar volume of liquid refrigerant would produce a lot more pressure, so I'll have to discuss this with my thermo prof, but it's something to take note of. Especially since I think get oil into that sort of system for lubrication would be a lot easier than using steam oil and trying to deal with emulsification.

Let me hear some thoughts on this from all the engineers that lurk on here

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